The trend over the last few decades has been to increase the functionality of integrated circuits by reducing lithographic dimensions following Moore's law. For less than a decade, it has also been feasible to add functionalities to integrated circuits using the possibilities offered by integration in the vertical dimension, this being referred to as 3-D integration. This increase in integration density in the three available dimensions of integrated circuits has had the direct effect of increasing power density, leading to an increase in transistor junction temperatures and, in general, electronic chip operating temperatures.
Thus, known integrated circuits comprise:
a substrate that extends mainly in a plane referred to as the “plane of the substrate”, this substrate being equipped with electrical connections for electrically connecting the integrated circuit to an exterior electronic circuit and having an interior side;
a top electronic chip and a bottom electronic chip, each of these electronic chips being electrically connected to the electrical connections by way of the substrate and each electronic chip comprising a front side turned toward the interior side of the substrate and a back side opposite the front side, the back side of the top electronic chip being located at a height H above the interior side of the substrate and being further from the interior side of the substrate than the back side of the bottom electronic chip; and
an encapsulation formed by a thick layer made of an electrically insulating material that coats the electronic chips, this thick layer forming an upper exterior side of the encapsulation parallel to the plane of the substrate.
Such integrated circuits are cooled via their upper exterior side. For example, for this purpose, a cooling system is fastened to this upper exterior side.
However, the bottom electronic chip is less well cooled than the top electronic chip because it is further from the upper exterior side. Under these conditions, to prevent the bottom electronic chip from becoming too hot, one solution consists in over-specifying the cooling system of the upper exterior side in order to take account of this distance to the bottom electronic chip. Another solution consists in doing nothing and in making allowance for the fact that the bottom electronic chip is less well cooled than the top electronic chip. Lastly, other solutions consist in inserting a heat sink, devoid of electronic components, between the back side of the bottom electronic chip and the upper exterior side of the encapsulation. This sink comprises a front side that is placed on the back side of the bottom electronic chip, and a back side opposite the front side, this back side being located on the same level as the back side of the top electronic chip. Examples of this last solution are described in patent applications US 2009/127700 A1 and JP 2013/008748 A. However, integration of a heat sink above the bottom electronic chip complexifies the fabrication of the integrated circuit.
Prior art is also known from: US 2013/043581 A1, EP 1 271 649 A2, US 2006/039118 A1 and US 2008/237840 A1.